154 GENERAL BIOLOGY 



cells are fixed in a normal condition. Usually some harden- 

 ing of the tissue takes place during the fixation, but if not 

 the alcohol used for preservation will also accomplish this 

 purpose. 



Killing and fixing reagents are numerous; some are good 

 for a special purpose only and others have a wider use. 

 Those mentioned among the reagents are good, easily used, 

 and satisfactory for any of the work demanded in a begin- 

 ning course. 



After fixation the tissue must be washed to remove the 

 excess of the reagent, and then preserved in 80 per cent 

 alcohol until wanted for use. Since this procedure has a 

 bleaching effect on the tissues, dyes are used to stain the 

 object to make it more easily seen. If sections are desired 

 it is necessary to embed the tissue or organ in paraffin or 

 other medium to support the softer mass while the sections 

 are being made. Directions for this must be had from books 

 on histological technic. 



For examination of living tissues normal salt solution is 

 the medium used; for fresh water animals, if alive, water 

 is used; preserved animals are examined in the solution 

 in which they are preserved. Glycerin is used if it is desired 

 to render the objects somewhat transparent. They may 

 be placed in the glycerin from water, alcohol, or formalin 

 directly. 



If a permanent mount is desired the object is usually 

 first stained, then dehydrated by placing for about an hour 

 in alcohol of various grades (35 per cent, 50 per cent, 70 

 per cent, 80 per cent, 95 per cent). If absolute alcohol is 

 at hand they should be placed in this for an hour to com- 

 plete the dehydration. To remove the alcohol some fluid, 

 usually an oil, is used which will mix with the alcohol and 

 also with the balsam which is used to complete the mounting. 



